The present invention relates to a projection exposure method for transferring a pattern of a mask onto a photosensitive substrate in a photolithography process for manufacturing semi-conductor elements, imaging elements (charge-coupled devices and the like), liquid crystal display elements or thin-film magnetic heads, which method is particularly applicable to a scanning exposure apparatus of step-and-scan type in which exposure is performed while scanning the mask and the photosensitive substrate are scanned in a synchronous manner.
As a related art, when a semi-conductor element is manufactured, a projection exposure apparatus for transferring a pattern of a reticle (as a mask) onto a wafer (or a glass plate or the like) on which photo-resist is coated through a projection optical system is used. Recently, in order to respond to a request that an area of a pattern to be transferred should become larger without making the projection optical system larger, there is proposed a projection exposure apparatus of step-and-scan type in which, after a wafer is stepped to a scan start position, by scanning a reticle and the wafer with respect to the projection optical system in a synchronous manner, a pattern image of the reticle is successively transferred onto shot areas on the wafer.
In the scanning exposure apparatus such as the projection exposure apparatus of step-and-scan type, since the pattern image of the reticle must be aligned with each shot area on the wafer with high accuracy during the exposure, there are provided a reticle alignment microscope for detecting positions of alignment marks on the reticle, and a wafer alignment sensor for detecting positions of alignment marks (wafer marks) on the wafer. When the reticle is exchanged for another one, the reticle is aligned with a wafer stage by using the reticle alignment microscope, and, when the wafer is exchanged for another one, the position of each shot area on the wafer is measured by using the wafer alignment sensor. Then, scanning exposure is performed in a condition that the pattern image of the reticle is overlapped with each shot area on the basis of the measured result.
In this case, for example, by detecting a position of a corresponding reference mark on the wafer by using the wafer alignment sensor while observing the corresponding alignment mark by using the reticle alignment microscope, a relative distance (i. e., base line amount) between a projection position (i. e., exposure center) of the reticle pattern image and a detection position (i. e., detection center) of the wafer alignment sensor is determined. A process for measuring the base line amount in this way is called as "base line check". By correcting the measured amount obtained by the wafer alignment sensor by the base line amount, the reticle pattern image can be overlapped with each shot area on the wafer with high accuracy.
In the above-mentioned scanning exposure apparatus, as mentioned above, the alignment between the reticle and each shot area on the wafer is performed by using the reticle alignment microscope and the wafer alignment sensor. However, for example, when one lot of wafers are successively exposed by using a single reticle, the reticle is thermally expanded gradually due to repeated illumination of exposure light illuminated onto the reticle, with the result that a length of the circuit pattern described on the reticle is gradually changed. Further, there is a danger of deviating the length of the circuit pattern from a design value due to the describing error. When it is assumed that the actual length of the circuit pattern described on the reticle is LR and the design length of the circuit pattern is LR.sub.0, a magnification error (%) of the reticle is represented as follows: EQU 100.times.(LR-LR.sub.0)/LR.sub.0
If such a magnification error of the reticle exceeds a limit of an allowable range, an error in overlap (hereinafter called "overlap error") between the reticle pattern image and each shot area (chip pattern) on the wafer will also exceed a limit of an allowable range, thereby worsening the yield of the semi-conductor elements finally obtained.
Regarding this, for example, by arranging a pair of reticle alignment microscopes along a non-scanning direction on the reticle, when the base line check is effected, the magnification error of the reticle in the non-scanning direction can be MEASURED by detecting a displacement amount of a distance between two alignment marks. If the magnification error in the non-scanning direction can be measured correctly in this way, the overlap error in the non-scanning direction can be reduced, for example, by adjusting projection magnification of the projection optical system accordingly.
Further, in the scanning exposure apparatus, if the magnification error of the reticle in a scanning direction can be measured correctly, the overlap error in the scanning direction can also be reduced, for example, by correcting a value of a ratio between a scanning speed of the reticle and a scanning speed of the wafer in accordance with the magnification error.
Thus, it could be considered that the magnification error of the reticle in the scanning direction is measured by arranging a pair of reticle alignment microscopes along the scanning direction on the reticle. However, since a projection area of the projection optical system used in the exposure apparatus of step-and-scan type is narrow (for example, slit) in the scanning direction, a distance between the pair of reticle alignment microscopes along the scanning direction must be so short corresponding to the narrow projection area, and it is therefore difficult to arrange them. Consequently, it is difficult to measure the magnification error in the scanning direction with high accuracy. Further, if additional reticle alignment microscope(s) for measuring the magnification error in the scanning direction is provided, in addition to the reticle alignment microscopes disposed along the non-scanning direction or in combination with the latter, the entire apparatus would be complicated and costly.
Further, since through-put (productivity) must be improved in the manufacturing process of the semi-conductor elements, it is desirable that the magnification error in the scanning direction is measured more efficiently.